It isn't hard to clock a plane. I like to use 2 cars, 2 cell phones or walkie talkies, 1 stop watch, 2 folding chairs and 2 helpers.
Measure off a distance...528 feet is a handy one.....and station both cars at the extreme ends. The cars provide protection and a point of reference in the fore ground that the observer can use to line up with his point of reference in the background.
The 2 observers can work out the procedure that works best for them, but we have the guy at the finish line hold the stop watch and the guy at the starting point tells the other guy when to start the clock.
After doing it this way for a number of years and dozens of clockings per outing, it's amazing how CONSISTENT the clockings are. Usually within a 1/100th of a second of each other on runs that sounded the same.
Once you have established a point of reference doing it this way, then your Wavoscope or radar gun readings become supplementary.